Step attenuator. This will be your primary signal level standard. Should have
at least 70 (preferably 90+) db range and be 50 ohm impedance.

HF general coverage receiver. This must cover the filter’s range of
frequencies. It should have a means of turning off the AGC and using manual RF
gain control. It will be your primary frequency standard, so a digital display
is recommended. It should have an analog S-meter.

(Optional) Audio AC voltmeter (or Scope) This can be used in place of the
receiver’s S-meter for better accuracy.

Equipment setup:

The basic setup is to feed the output of the RF generator thru the Step
attenuator into the filter. The filter output is fed into the HF receiver. If
you are using the Audio voltmeter, connect it to the receiver’s speaker output.

Filter Termination:

For best results, the input and output impedance of the filter should be
matched according to the filter specs. This may be difficult as the filter
impedance is not always known. In some cases it may be printed right on the
filter. Also, some filters are designed to have their inputs and outputs tuned
with a small value capacitor. The filter impedances can be matched to 50 ohms
by using a series resistance.

Example: Heathkit filters have Zin = Zout = 2000 ohms. The RF generator has a
50 ohm output impedance. Use 1950 ohms in series with input and output so
the filter ‘sees’ 2K ohms on both ports.

Also, a good ground into and out of the filter is required to get accurate
results. A small impedance in the ground can lead to erroneously high stopband
‘blowby’.

In Site Measurements:

The best results can be had by using the filter in the rig where it will
be used (called Rig Under Test or RUT in the following). This is because the
termination and grounding issues are taken care of for you. In this case, you
can connect the RF generator to the input of the RUT and set it to any
frequency the RUT can receive. The RUT will convert the RF generator frequency
to the passband range of the filter. Connect the HF receiver used for
measurement to a downstream stage to pick off a signal after the filter. Be
sure to disable the AGC in the RUT, and set the RF generator output level and
RUT RF gain control so the RUT is not overloaded.

Measurement technique:

The HF receiver should be in CW or SSB mode, using as wide a bandwidth as
possible. Tune the generator and HF receiver to peak up a signal and find the
peak of the filter’s passband. Set the step attenuator to around 70 or 80 dB of
attenuation. Adjust the generator output so you have a reading of S9 or so on
the receiver.

The procedure for plotting the filter response is to make a series of signal
level measurements at the frequencies of interest. These measurements will be
relative to a single reference, so you will be able to see the filter’s
passband characteristic but not it’s absolute insertion loss.

There are two basic ways to make these relative measurements.

The first is using the S-meter of the HF receiver to indicate a fixed signal
level. For this technique, the AGC in the HF receiver is on. At the filter
peak, record the S-meter reading. This is your ‘reference level’. Next set the
RF generator to the frequency you wish to measure. You can determine exact
frequency by zero beating and reading the digital display from the HF receiver.
Each time you change the RF generator, retune the HF rig to give the same beatnote
(i.e. the HF receiver must track the generator frequency). This also prevents
the filter characteristics of the HF receiver from affecting the results – all
the measurements are at the same point in the HF receiver’s IF. As you move the
RF generator signal away from the peak, the S-meter reading will decrease.
Keeping the RF generator fixed at the frequency where you want to make a
measurement, decrease the attenuation on the step attenuator until the S-meter
comes back up to the recorded reading ‘reference level’. Now you can read the
change in filter attenuation by noting the change you made in the step
attenuator setting.

The second method is to use an audio AC voltmeter instead of the S-meter to
measure the reference level. In this case, turn off the AGC for the HF receiver
and use manual RF gain control to make sure the HF receiver is not overloaded.
Measure the audio output from the receiver with the AC voltmeter. This is your
‘reference level’. Change the RF generator and HF receiver frequency together.
Next adjust the step attenuator to bring the level back up to the reference
level. Now read the filter attenuation on the step attenuator as above.

Caveats:

At the extremes of measurement, you need to be sure you are not overloading the
HF receiver (or the RUT). Do this by making a 3-6 db decrease in the step
attenuator, and checking that signal levels at the measurement point increase
appropriately. If they don’t then you may need to readjust the RF generator and
start the measurements over with a new ‘reference level’.

Another Way

A very convenient way to measure filters is using a DSP receiver such as the
756 Pro series with a PC equipped with a sound card and audio spectral anaylsis
software (shareware). A good program to use is Spectrogram from Visualization
Software. The sound card software is used to measure and plot the audio output
from the HF receiver. As long as the bandwidth of the HF receiver is wider than
the filter you want to test, you can easily sweep the input to the filter while
observing the output on the computer screen. In this technique, you need to
turn off the AGC in the HF receiver so it doesn’t affect the signal level
measured by the sound card.

This technique could also be used to measure the
passband of a RUT directly, as long as the AGC can be turned off.